This invention relates to a method for retarding the setting time of rapid hardening cement manufactured from clinker which contains 11CaO.7Al.sub.2 O.sub.3.CaX.sub.2 (X represent a halogen atom) and solid solutions of 3CaO.SiO.sub.2, 2CaO.SiO.sub.2 and 4CaO.Al.sub.2 O.sub.3.Fe.sub.2 O.sub.3.
Cement containing 11CaO.7Al.sub.2 O.sub.3.CaX.sub.2 shows instantaneous setting, and lower age strength development at short and longer periods, and therefore it had no practical use. Cement prepared by adding insoluble anhydrite (CaSO.sub.4) to the clinker powder in an amount such that the weight ratio of Al.sub.2 O.sub.3 /SO.sub.3 in the cement is present in a range of 0.7-1.8 or by adding insoluble anhydrite to the clinker in the same amount as the above and intergrinding the mixture shows high strength development, but the setting time thereof is too short like 10-15 minutes.
The research group of the present inventors has investigated to retard the setting time of cement prepared from the clinker and invented that (1) cement obtained by adding insoluble anhydrite to the clinker powder or intergrinding a mixture of insoluble anhydrite and the clinker in an amount such that the weight ratio of Al.sub.2 O.sub.3 /SO.sub.3 in the cement is present in a range of 0.7-1.8, shows retarded setting time, (2) cement having retarded setting time, excellent workability and increased initial and longer age strengths can be obtained by adding insoluble anhydrite or a mixture of insoluble anhydrite and gypsum hemihydrate (CaSO.sub.4.1/2 H.sub.2 O) to the clinker in an amount such that the weight ratio of the mixture is present in a range of 0.7-1.8 and then mixing at least one compound selected from the group consisting of sulphates, nitrates and chlorides of potassium, sodium, magnesium, calcium, aluminium and ammonium (exclusive of insoluble anhydrite and gypsum hemihydrate) with mixing water (U.S. Pat. application Ser. No. 211,307, now U.S. Pat. No. 3,782,992), (3) cement having retarded setting time can be obtained by adding insoluble anhydrite and at least one compound such as saccharides, sodium hydrogen carbonate, water soluble phosphates, aliphatic carboxylic acids, fluorosilicates, sodium silicates, lignin sulphonic acid, sulphuric acid esters of higher alcohols and alkyl sulphonate to the clinker wherein the anhydrite is present in an amount such that the weight ratio of Al.sub.2 O.sub.3 /SO.sub.3 in the cement is 0.7-1.8 (U.S. application Ser. No. 253,352, now U.S. Pat. No. 3,864,138) and (4) cement having shortened setting time even at low temperature is prepared by adding at least one carbonate such as calcium carbonate, sodium carbonate, potassium carbonate and magnesium carbonate or a mixture of at least one carbonate as shown above and at least one sulphate such as calcium sulphate, sodium sulphate, potassium sulphate and magnesium sulphate to the clinker in an amount such that the weight ratio of Al.sub.2 O.sub.3 /SO.sub. 3 in the cement is present in a range of 0.6-1.8 (U.S. application Ser. No. 309,805, now U.S. Pat. No. 3,819,389).
One object of the present invention is to manufacture cement wherein the setting time thereof is retarded longer than that of cement produced from clinker containing 11CaO.7Al.sub.2 O.sub.3.CaX.sub.2 in the prior art and the high strength at short and longer periods is developed.
This object and other objects of the present invention as hereinafter will become more readily apparent can be attained by adding at least a boric compound such as boric acid, boric acid anhydride and borates such as borax to the mixture of the clinker and anhydrite or by adding the boric compound to the cements produced by the above process 1-4. Especially the setting time of cement manufactured by adding the boric compound to the cement produced by the above process 4 can be controlled at higher temperature and even at lower temperature.
The results of the experiments are illustrated as follows.
The clinker containing 11CaO.7Al.sub.2 O.sub.3.CaF.sub.2 was obtained by mixing powders of clay, bauxite, lime stone and fluorspar, intergrinding the mixture, granulating it and firing the granules in a rotary kiln at 1300.degree.-1350.degree.C. The chemical and mineral compositions of the clinker thus obtained are shown in Table 1.
Table 1 __________________________________________________________________________ Chemical Composition (%) Mineral Composition (%) __________________________________________________________________________ SiO.sub.2 Al.sub.2 O.sub.3 Fe.sub.2 O.sub.3 CaO MgO Total 11CaO.7Al.sub.2 O.sub.3.CaF.sub.2 3CaO.SiO.sub.2 17.3 14.0 2.5 61.9 1.0 96.7 24.5 50.7 __________________________________________________________________________
Insoluble anhydrite was prepared by firing phosphogypsum at 800.degree.-900.degree.C, grinding the fired gypsum to a specific surface area of about 8100 cm.sup.2 /g (Blaine). The chemical composition of the insoluble anhydrite thus obtained is shown in Table 2.
Table 2 ______________________________________ Chemical composition of anhydrite (%) ______________________________________ CaO SO.sub.3 SiO.sub.2 +R.sub.2 O P.sub.2 O.sub.5 F Total 36.0 54.4 5.9 3.42 0.23 99.95 ______________________________________